how did the king and the duke fool the people in the various villages they visited?

The Duke and the King are charlatans. They enjoy thinking they have tricked people and then they like to take advantage of them. The King and Duke prey on people's ignorance whether they are holding a religious revival or a Shakespearean play. Huck is pretty street-wise. He knows that beneath their jovial lies, these guys can turn nasty and cutthroat. Huck knows it is safer to humour them than offend them.